In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
In LTE a user equipment in a connected mode, such as Radio Resource Control (RRC)_CONNECTED mode, measures a signal strength, e.g. Reference Symbol Received Power (RSRP), or signal quality, e.g. Reference Symbol Received Quality (RSRQ), of the serving cell and neighboring cells. The user equipment reports measurement results in a measurement report when measurements fulfill an event criterion defined by the radio base station. One event criterion, also referred to as event A3, is fulfilled when a neighbor cell gets a certain amount, a threshold, stronger than the serving cell for at least a certain minimum time. This event criterion is typically used as trigger for handover, i.e. a handover of the user equipment is initiated by the radio base station to the neighbor cell that fulfills the event criterion and have enough resources to support the UE connection. There might be cases when several neighbor cells fulfill this criterion. If this happens the user equipment sends a list including those cells to the radio base station.
The tuning, also called handover tuning, of the thresholds used for triggering handover is determined making the handover as early as possible but still late enough to avoid handover back again. This is accomplished by tuning the handover margin to a suitable compromise that avoids too many and frequent handovers, such as oscillating handovers. The handover tuning is for simplicity typically done on network level, e.g. radio base station in LTE, a base station controller in GSM, or a radio network controller in WCDMA, and may consider parameters such as large and small cells, fast and slow moving user equipments, and varying propagation conditions e.g. street level, indoor or open areas.
Increasing handover margin will mitigate too frequent handovers but will also cause delayed handover initialization, thereby degrading the throughput before the handover is triggered and eventually also increased drop rate. Increasing filtering of the measured quantity and time to trigger, e.g. Layer 3 (L3) filtering, used for the event evaluation will also mitigate too frequent handovers and unfortunately also cause delayed handover initialization but not to the same extent as increased handover margin.
Known solutions for reducing handover frequency in average and still avoiding the drawbacks are to adjust the handover margin for a specific user equipment. This is typically based on recognizing that a user equipment need to do handover soon again after just having performed handover and that the indicated target is the most recent visited cell, thus detecting an oscillating handover and adjust the handover margin for that specific user equipment temporarily as an effort to mitigate the detected oscillation tendencies. This, however, requires a large amount of computational capacity as well as radio resources and is a complicated process.